Project Summary/Abstract: The overall goal of this project is to define the mechanisms that are used to generate diverse muscle fiber types. In animals, different muscle fiber types arise to fine-tune the functions of individuals muscles, to enable those muscles to carry out their distinct roles in the animal. However, there is still much to learn of the cellular and molecular mechanisms that direct groups of myoblasts towards one fiber fate versus others. We have demonstrated that the flight muscles of the adult Drosophila thorax are specified and differentiate using mechanisms that are conserved with vertebrate fast fiber specification. Moreover, we have identified a number of transcriptional regulators that are required for formation of the flight muscles or of the physiologically-distinct jump muscles. In this renewal, we will build upon our studies to define conserved signaling and transcriptional pathways that give rise to different adult muscle fiber types. In the first aim, we will determine how ectodermal signals such as Wingless specify flight muscle specification via Vestigial and the flight muscle identity factor Spalt-major. In the second aim, we will define the roles of orthologs of mammalian Mohawk and Sox6 in regulating muscle fiber identity. In the third aim, we will focus upon how Vestigial and its cofactor Scalloped regulate jump muscle differentiation. Overall, our studies will provide significant new insight into how different muscle fiber types are generated during development in Drosophila. Importantly, our published and preliminary data provide significant evidence that conserved mechanisms regulate muscle fiber fate choices, indicating that our findings can be used as a valuable framework for understanding vertebrate muscle specification. Moreover, since muscle diseases and diabetes often affect the prevalence of one muscle fiber type over another, understanding the normal mechanisms of muscle fiber specification and maintenance can provide insight into the diseased state and how those conditions might be ameliorated.